clang build: unsequenced modifications.

[stolk]

When building with clang:

/home/bram/src/openal-soft/jni/OpenAL/Alc/opensles.c:239:24: warning: multiple unsequenced modifications to 'bufferIndex'
[-Wunsequenced]
bufferIndex = (++bufferIndex) % bufferCount;
~ ^
/home/bram/src/openal-soft/jni/OpenAL/Alc/opensles.c:368:20: warning: multiple unsequenced modifications to 'bufferIndex'
[-Wunsequenced]
bufferIndex = (++bufferIndex) % bufferCount;

A more straightforward:
bufferIndex = ( bufferIndex+1) % bufferCount;
...should be used.

bram